Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate.
Currently, most commercial lithographic plates require a development process after the plates being exposed and before put on press. A liquid developer is used to dissolve and clean off the non-exposed areas (for negative plate) or the exposed areas (for positive plate). On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial printing operation and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing savings on labor, material, and developer waste disposal costs. Among the on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,407,764, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, and 6,482,571.
Traditionally the plate is exposed with an actinic light (usually an ultraviolet light from a lamp) through a separate photomask having predetermined image pattern that is placed between the light source and the plate. Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser, allowing the elimination of photomask, reducing material, equipment and labor cost. Among the laser imagable plates, infrared laser sensitive plates, also called thermosensitive plates or thermal plates, are most attractive because they often can be handled under white light.
Negative thermosensitive lithographic printing plates having on a substrate a thermosensitive layer comprising a polymeric binder, an ethylenically unsaturated monomer, an initiator, and an infrared absorbing dye have been described in the literature, such as U.S. Pat. Nos. 4,997,745, 6,153,356, 6,232,038, 6,309,792, and 6,645,697. As the ethylenically unsaturated monomer, multifunctional (meth)acrylate monomer is generally used, including urethane (meth)acrylate monomers and non-urethane (meth)acrylate monomers. Urethane (meth)acrylate monomers generally give better curing speed and press durability than non-urethane (meth)acrylate monomers. Multifunctional urethane (meth)acrylate monomer has been used either alone (as in U.S. Pat. No. 6,232,038), blended with a multifunctional non-urethane (meth)acrylate monomer at a urethane (meth)acrylate monomer to non-urethane (meth)acrylate monomer weight ratios of at least 3.5 (for example, 3.5 to 4.9 as in U.S. Pat. No. 6,309,792, and 4.0 to 4.8 as in U.S. Pat. App. Pub. No. 2002/0197564), or blended with a difunctional non-urethane (meth)acrylate monomer (such as in U.S. Pat. No. 6,153,356).
The inventor has found, surprisingly, thermosensitive lithographic printing plates comprising a multifunctional urethane (meth)acrylate monomer having at least 6 (meth)acrylate groups and a multifunctional non-urethane (meth)acrylate monomer having at least 4 (meth)acrylate groups at a weight ratio of from 0.10 to 3.0 can give significantly better curing speed and press durability than with such multifunctional urethane (meth)acrylate monomer or such multifunctional non-urethane (meth)acrylate monomer alone.